Sphingosine 1-phosphate (S1P) is a lysophospholipid mediator that evokes a variety of cellular responses by stimulation of five members of the endothelial cell differentiation gene (EDG) receptor family. The EDG receptors are G-protein coupled receptors (GPCRs) and on stimulation propagate second messenger signals via activation of heterotrimeric G-protein alpha (Gα) subunits and beta-gamma (Gβγ) dimers. Ultimately, this S1P-driven signaling results in cell survival, increased cell migration and, often, mitogenesis. The recent development of agonists targeting S1P receptors has provided insight regarding the role of this signaling system in physiologic homeostasis. For example, the immunomodulating agent, FTY720 (2-amino-2-[2-(4-octylphenyl)ethyl]propane 1,3-diol), that following phosphorylation, is an agonist at 4 of 5 S1P receptors, revealed that affecting S1P receptor activity influences lymphocyte trafficking. Further, S1P type 1 receptor (S1P1) antagonists cause leakage of the lung capillary endothelium, which suggests that S1P may be involved in maintaining the integrity of the endothelial barrier in some tissue beds. S1P type 4 receptors (S1P4) are expressed mainly in leukocytes, and specifically S1P4 mediates immunosuppressive effects of S1P by inhibiting proliferation and secretion of effector cytokines, while enhancing secretion of the suppressive cytokine IL-10. See, for example, Wang, W. et. al., (2005) FASEB J. 19(12): 1731-3, which is incorporated by reference in its entirety. S1P type 5 receptors (S1P5) are exclusively expressed in oligodendrocytes and oligodendrocyte precursor cells (OPCs) and are vital for cell migration. Stimulation of S1P5 inhibits OPC migration, which normally migrate considerable distances during brain development. See, for example, Novgorodov, A. et al., (2007) FASEB J, 21: 1503-1514, which is incorporated by reference in its entirety.
S1P has been demonstrated to induce many cellular processes, including those that result in platelet aggregation, cell proliferation, cell morphology, tumor-cell invasion, endothelial cell chemotaxis and angiogenesis. For these reasons, S1P receptors are good targets for therapeutic applications such as wound healing, tumor growth inhibition, and autoimmune diseases.
Sphingosine-1-phosphate signals cells in part via a set of G protein-coupled receptors named S1P1, S1P2, S1P3, S1P4, and S1P5 (formerly EDG1, EDG5, EDG3, EDG6 and EDG8). The EDG receptors are G-protein coupled receptors (GPCRs) and on stimulation propagate second messenger signals via activation of heterotrimeric G-protein alpha (Gα) subunits and beta-gamma (Gβγ) dimers. These receptors share 50-55% amino acid sequence identity and cluster with three other receptors (LPA1, LPA2, and LPA3 (formerly EDG2, EDG4 and EDG7) for the structurally related lysophosphatidic acid (LPA).
A conformational shift is induced in the G-Protein Coupled Receptor (GPCR) when the ligand binds to that receptor, causing GDP to be replaced by GTP on the α-subunit of the associated G-proteins and subsequent release of the G-proteins into the cytoplasm. The α-subunit then dissociates from the βγ-subunit and each subunit can then associate with effector proteins, which activate second messengers leading to a cellular response. Eventually the GTP on the G-proteins is hydrolyzed to GDP and the subunits of the G-proteins reassociate with each other and then with the receptor. Amplification plays a major role in the general GPCR pathway. The binding of one ligand to one receptor leads to the activation of many G-proteins, each capable of associating with many effector proteins leading to an amplified cellular response.
S1P receptors make good drug targets because individual receptors are both tissue and response specific. Tissue specificity of the S1P receptors is desirable because development of an agonist or antagonist selective for one receptor localizes the cellular response to tissues containing that receptor, limiting unwanted side effects. Response specificity of the S1P receptors is also of importance because it allows for the development of agonists or antagonists that initiate or suppress certain cellular responses without affecting other responses. For example, the response specificity of the S1P receptors could allow for an S1P mimetic that initiates platelet aggregation without affecting cell morphology.
Sphingosine-1-phosphate is formed as a metabolite of sphingosine in its reaction with sphingosine kinase and is stored in abundance in the aggregates of platelets where high levels of sphingosine kinase exist and sphingosine lyase is lacking. S1P is released during platelet aggregation, accumulates in serum, and is also found in malignant ascites. Reversible biodegradation of S1P most likely proceeds via hydrolysis by ectophosphohydrolases, specifically the sphingosine-1-phosphate phosphohydrolases. Irreversible degradation of S1P is catalyzed by S1P lyase yielding ethanolamine phosphate and hexadecenal.
Autotaxin (ATX, ENPP2) is a secreted glycoprotein widely present in biological fluids, including blood, cancer ascites, synovial, pleural and cerebrospinal fluids, originally isolated from the supernatant of melanoma cells as an autocrine motility stimulation factor (Stracke, M. L., et al. Identification, purification, and partial sequence analysis of autotaxin, a novel motility-stimulating protein. J Biol Chem 267, 2524-2529 (1992), which is incorporated by reference in its entirety). ATX is encoded by a single gene on human chromosome 8 (mouse chromosome 15) whose transcription, regulated by diverse transcription factors (Hoxal3, NFAT-1 and v-jun), results in four alternatively spliced isoforms (α, β, γ, and δ). See, for example, Giganti, A., et al Murine and Human Autotaxin alpha, beta, and gamma Isoforms: Gene organization, tissue distribution and biochemical characterization. J Biol Chem 283, 7776-7789 (2008); and van Meeteren, L. A. & Moolenaar, W. H. Regulation and biological activities of the autotaxin-LPA axis. Prog Lipid Res 46, 145-160 (2007); Hashimoto, et al, “Identification and Biochemical Charaterization of a Novel Autotaxin Isoform, ATXδ,” J. of Biochemistry Advance Access (Oct. 11, 2011); each of which is incorporated by reference in its entirety.
ATX is synthesized as a prepro-enzyme, secreted into the extracellular space after the proteolytic removal of its N-terminal signal peptide (Jansen, S., el al Proteolytic maturation and activatio of autotaxin (NPP2), a secreted metastasis-enhancing lysophospho lipase D. J Cell Sci 118, 3081-3089 (2005), which is incorporated by reference in its entirety). ATX is a member of the ectonucleotide pyrophosphatase/phosphodiesterase family of ectoenzymes (E-NPP) that hydrolyze phosphodiesterase (PDE) bonds of various nucleotides and derivatives (Stefan, C, Jansen, S. & Bollen, M. NPP-type ectophosphodiesterases: unity in diversity. Trends Biochem Sci 30, 542-550 (2005), which is incorporated by reference in its entirety). The enzymatic activity of ATX was enigmatic, until it was shown to be identical to lysophospholipase D (lysoPLD) (Umezu-Goto, M., et al. Autotaxin has lysophospholipase D activity leading to tumor cell growth and motility by lysophosphatidic acid production. J Cell Biol 158, 227-233 (2002), which is incorporated by reference in its entirety), which is widely present in biological fluids. Since ATX is a constitutively active enzyme, the biological outcome of ATX action will largely depend on its expression levels and the local availability of its substrates. The major lysophospholipid substrate for ATX, lysophosphatidylcholine (LPC), is secreted by the liver and is abundantly present in plasma (at about 100 μM) as a predominantly albumin bound form (Croset, M., Brossard, N., Polette, A. & Lagarde, M. Characterization of plasma unsaturated lysophosphatidylcholines in human and rat Biochem J 345 Pt 1, 61-67 (2000), which is incorporated by reference in its entirety). LPC is also detected in tumor-cell conditioned media (Umezu-Goto, M., et al.), presumably as a constituent of shed microvesicles. ATX, through its lysoPLD activity converts LPC to lysophosphatidic acid (LPA).
LPC is an important inflammatory mediator with recognized effects in multiple cell types and pathophysiological processes. It is a major component of oxidized low density lipoprotein (oxLDL) and it can exist in several other forms including free, micellar, bound to hydrophobic proteins such as albumin and incorporated in plasma membranes. It is produced by the hydrolysis of phosphatidylcholine (PC) by PLA2 with concurrent release of arachidonic acid and in turn of other pro-inflammatory mediators (prostaglandins and leukotrienes). Moreover, LPC externalization constitutes a chemotactic signal to phagocytic cells, while interaction with its receptors can also stimulate lymphocytic responses. LPC has been shown to have therapeutic effects in experimental sepsis, possibly by suppressing endotoxin-induced HMGB1 release from macrophages/monocytes.
LPA, the product of ATX action on LPC, is a bioactive phospholipid with diverse functions in almost every mammalian cell line (Moolenaar, W. H., van Meeteren, L. A. & Giepmans, B. N. The ins and outs of lysophosphatidic acid signaling. Bioessays 28, 870-881 (2004), which is incorporated by reference in its entirety). LPA is a major constituent of serum bound tightly to albumin, gelsolin and possibly other as yet unidentified proteins. See, e.g., Goetzl, E. J., et al Gelsolin binding and. cellular presentation of lysophosphatidic acid. J Biol Chem 275, 14573-14578 (2000); and Tigyi, G. & Miledi, R, Lysophosphatidates bound to serum albumin activate membrane currents in Xenopus oocytes and neurite retraction in PC12 pheochromocytoma cells. J Biol Chem 267, 21360-21367 (1992); each of which is incorporated by reference in its entirety.
LPA is also found in other biofluids, such as saliva and follicular fluid, and has been implicated in a wide array of functions, such as wound heeling, tumor invasion and metastasis, neurogenesis, myelination, astrocytes outgrowth and neurite retraction. The long list of LPA functions was also explained with the discovery that it signals through G-protein coupled receptors (GPCRs), via classical second messenger pathways. Five mammalian cell-surface LPA receptors have been identified so far. The best known are LPA1-3 (namely Edg-2, Edg-4 and Edg7) which are all members of the so-called ‘endothelial differentiation gene’ (EDG) family of GPCRs (Contos, J. J., Ishii, I. & Chun, J. Lysophosphatidic acid receptors. Mol Pharmacol 58, 1188-1196 (2000), which is incorporated by reference in its entirety). LPA receptors can couple to at least three distinct G proteins (Gq, Gi and G12/13), which, in turn, feed into multiple effector systems. LPA activates Gq and thereby stimulates phospholipase C (PLC), with subsequent phosphatidylinositol-bisphosphate hydrolysis and generation of multiple second messengers leading to protein kinase C activation and changes in cytosolic calcium. LPA also activates Gi, which leads to at least three distinct signaling routes: inhibition of adenylyl cyclase with inhibition of cyclic AMP accumulation; stimulation of the mitogenic RAS-MAPK (mitogen-activated protein kinase) cascade; and activation of phosphatidylinositol 3-kinase (PI3K), leading to activation of the guanosine diphosphate/guanosine triphosphate (GDP/GTP) exchange factor TIAM1 and the downstream RAC GTPase, as well as to activation of the AKT/PKB antiapoptotic pathway. Finally, LPA activates G12/13, leading to activation of the small GTPase RhoA, which drives cytoskeletal contraction and cell rounding. So, LPA not only signals via classic second messengers such as calcium, diacylglycerol and cAMP, but it also activates RAS- and RHO-family GTPases, the master switches that control cell proliferation, migration and morphogenesis.
LPA signaling through the RhoA-Rho kinase pathway mediates neurite retraction and inhibition of axon growth. Interfering with LPA signaling has been shown to promote axonal regeneration and functional recovery after CNS injury or cerebral ischemia. (See Broggini, et al., Molecular Biology of the Cell (2010), 21:521-537.) It has been reported that addition of LPA to dorsal root fibers in ex vivo culture causes demyelination, whereas LPC fails to cause significant demyelination of nerve fibers in ex vivo cultures without further addition of recombinant ATX to the culture which when added caused significant demyelination at equivalent levels to LPA presumable due to conversion of LPC to LPA through the enzymatic activity of ATX. Moreover, injury induced demyelination was attenuated by about 50% in atx+/− mice (Nagai, et al., Molecular Pain (2010), 6:78).
A number of diseases or disorders involve demyelination of the central or peripheral nervous system which can occur for a number of reasons such as immune dysfunction as in multiple sclerosis, encephalomyelitis, Guillain-Barre Syndrome, chronic inflammatory demyelinating polyneuropathy (CIDP), transverse myelitis, and optic neuritis; demyelination due to injury such as spinal cord injury, traumatic brain injury, stroke, acute ischemic optic neuropathy, or other ischemia, cerebral palsy, neuropathy (e.g. neuropathy due to diabetes, chronic renal failure, hypothyroidism, liver failure, or compression of the nerve (e.g. in Bell's palsy)), post radiation injury, and central pontine myelolysis (CPM); inherited conditions such as Charcot-Marie-Tooth disease (CMT), Sjogren-Larsson syndrome, Refsum disease, Krabbe disease, Canavan disease, Alexander disease, Friedreich's ataxia, Pelizaeus-Merzbacher disease, Bassen-Kornzweig syndrome, metachromatic leukodystrophy (MLD), adrenoleukodystrophy, and nerve damage due to pernicious anemia; viral infection such as progressive multifocal leukoencephalopathy (PML), Lyme disease, or tabes dorsalis due to untreated syphilis; toxic exposure due to chronic alcoholism (which is a possible cause of Marchiafava-Bignami disease), chemotherapy, or exposure to chemicals such as organophosphates; or dietary deficiencies such as vitamin B12 deficiency, vitamin E deficiency and copper deficiency. Other demyelination disorders may have unknown causes or multiple causes such as trigeminal neuralgia, Marchiafava-Bignami disease and Bell's palsy. One particularly successful approach to treating demyelination disorders which are caused by autoimmune dysfunction has been to attempt to limit the extent of demyelination by treating the patient with immunoregulatory drugs. However, typically this approach has merely postponed but not avoided the onset of disability in these patients. Patients with demyelination due to other causes have even fewer treatment options. Therefore, the need exists to develop new treatments for patients with demyelination diseases or disorders.